The present invention relates to a toner and an image forming method used in a recording method utilizing electrophotography, electrostatic recording, magnetic recording, toner jet recording, etc. More particularly, the present invention relates to a toner used in an image forming method for an image forming apparatus, such as a copying apparatus, wherein a toner image is once formed on an electrostatic image-bearing member and then transferred onto a transfer-receiving material to form an image thereon, and an image forming method using the toner.
Hitherto, a large number of electrophotographic processes have been known. Generally, in these processes, an electrostatic latent image is formed on an electrostatic image-bearing member (hereinafter represented by a xe2x80x9cphotosensitive memberxe2x80x9d) utilizing ordinarily a photoconductive material, the latent image is then developed with a toner to form a visible toner image, and the toner image, after being transferred as desired onto a transfer-receiving material such as paper, is fixed onto the transfer-receiving material by application of pressure, heat, etc., to provide a product copy or print. As a method for visualizing the electrostatic latent image, there have been known the cascade developing method, the magnetic brush developing method, the jumping developing method, the pressure developing method, etc.
U.S. Pat. No. 3,909,258 has proposed a developing method using a magnetic toner having an electroconductivity. More specifically, in the developing method, an electroconductive magnetic toner carried on a hollow cylindrical electroconductive sleeve with a magnet installed inside thereof is caused to contact an electrostatic image to develop the image. In this instance, at the developing region, an electroconductive path is formed of the toner particles between the electrostatic image-bearing member and the sleeve surface, and the toner particles are supplied with a charge via the electroconductive path, whereby the toner particles are attached to the electrostatic image based on a Coulomb force acting between the charge and the electrostatic image. The developing method using an electroconductive magnetic toner is an excellent method obviating problems accompanying the conventional two-component developing method, but as the toner is electroconductive, the method is accompanied with a difficulty in electrostatically transferring the developed toner image from the electrostatic image-bearing member to a transfer-receiving material (or recording material) such as plain paper.
As a developing method using a high-resistivity magnetic toner allowing electrostatic transfer, one utilizing dielectric polarization of toner particles is known. Such a developing method however essentially involves problems, such as slow developing speed and insufficient developed image density, so that the commercialization is difficult.
As another developing method using a high-resistivity insulating magnetic toner, there is known a method wherein toner particles are triboelectrically charged through friction between individual toner particles and between toner particles and a friction member such as a sleeve, and the thus-charged toner particles are caused to contact an electrostatic image-bearing member to effect a development. This method is however accompanied with a problem that the triboelectric charge is liable to be insufficient due to few opportunities of contact between the toner particles and the friction member and much magnetic material exposed to the surfaces of the magnetic toner particles, leading to inferior images due to the insufficient charge.
As another developing method, Japanese Laid-Open Patent Application (JP-A) 54-43027 and JP-A 55-18656, for example, disclose a so-called jumping developing method wherein a magnetic developer (toner) is applied in a thin layer on a developer-carrying member to be triboelectrically charged thereon, and the charged layer of the magnetic toner is moved under the action of a magnetic field to be opposed in close proximity to but free of contact with an electrostatic latent image to effect a development. According to this method,the magnetic developer is allowed to be sufficiently triboelectrically charged by application in a thin layer on the developer-carrying member, and the developer carried under a magnetic force is used for development in a state free from contact with the electrostatic latent image, so that a high definition image can be obtained with suppression of so-called xe2x80x9cfogxe2x80x9d caused by transfer of the developer onto non-image parts.
Such a mono-component developing method, does not require carrier particles, such as glass beads or iron powder, so that a developing device therefor can be small-sized and light in weight. Further, while the two-component developing scheme requires devices for detecting a toner concentration in the developer and for replenishing a necessary amount of toner based on the detected result in order to keep a constant toner concentration in the developer, the mono-component developing scheme does not require such devices, thus allowing a small-sized and light developing device also from these points.
However, the developing method using an insulating magnetic toner involves an unstable factor a ttributable to the use of the insulating magnetic toner. This arises from the feature that a substantial amount of fine powdery magnetic material is contained in dispersion within the insulating magnetic toner particles and a portion of the magnetic material is exposed to the toner particle surfaces to affect the flowability and the triboelectric chargeability of the magnetic toner, thereby causing a change or deterioration of properties required of the magnetic toner, such as developing performance and continuous image forming performance.
The above-mentioned problems accompanying the use of a conventional magnetic toner containing a magnetic material is considered to be principally caused by the exposure of a magnetic material to the magnetic toner particle surface. More specifically, as a result of exposure of fine particles of magnetic material having a lower resistivity than a toner biner principally constituting the toner to the toner article surfaces, various difficulties are caused, such as a lowering in toner chargeability, a lowering in toner flowability, and developer deteriorations during a long term of use, such as peeling-off of the magnetic particles due to friction between individual toner particles and toner particles and the regulating member resulting in image density lowering and occurrence of density irregularity called xe2x80x9csleeve ghostxe2x80x9d.
Hitherto, various proposals have been made regarding magnetic iron oxide contained in magnetic toners, but room for improvement has yet been left.
For example, JP-A 62-279352 has proposed a magnetic toner containing silicon-containing magnetic iron oxide. The magnetic iron oxide is intentionally caused to contain silicon inside thereof, but the magnetic toner containing the magnetic iron oxide has left room for improvement regarding the flowability.
Japanese Patent Publication (JP-B) 3-9045 has proposed to provide magnetic iron oxide particles with a controlled spherical shape by adding a silicate salt thereto. The magnetic iron oxide particles obtained according to this proposal are caused to contain much silicon at an inner portion thereof and little silicon at the surface due to the use of a silicate salt for particle shape control and have a high surface smoothness. As a result, the resultant magnetic toner is provided with an improved flowability to some extent, but the adhesion between the toner binder resin and the magnetic iron oxide particles is liable to be insufficient.
JP-A 61-34070 has proposed a process for producing triiron tetroxide by adding a hydroxysilicate salt solution during oxidation to triiron tetroxide. The triiron tetroxide particles produced by the process contain Si in proximity to the surfaces thereof but are also caused to have a layer of Si in proximity to the surface thereof, so that the surface thereof is weak against a mechanical impact as by abrasion.
On the other hand, a toner has been conventionally produced through a (pulverization) process wherein a binder resin, a colorant (inclusive of a magnetic material in the case of a magnetic toner), etc., are melt-mixed for uniform dispersion, and then the mixture is pulverized by a pulverizer, and classified into toner particles having a prescribed particle size. This process however poses a restriction in material selection for complying with a recent trend for requiring a smaller particle size toner. For example, the resin-colorant dispersion mixture has to be sufficiently fragile so as to allow pulverization by a commercially feasible apparatus. If the resin-colorant dispersion mixture is sufficiently fragile for complying with the requirement, a practical high-speed pulverization of the resin-colorant dispersion mixture is liable to result in toner particles of a broad particle size range, particularly including a relatively large proportion of fine particle fraction (over-pulverized particles). Further, a toner composed of such a highly fragile material is subject to further pulverization or powder formation in copying apparatus, etc.
Further, according to the pulverization process, it is difficult to completely uniformly disperse solid fine particles of a magnetic material or a colorant in a resin, and the insufficient dispersion can lead to increased fog or lower image density while depending on the degree of the insufficiency. Further, the pulverization process essentially causes exposure of the magnetic iron oxide particles to the toner particle surfaces, thus inevitably leaving problems regarding toner flowability or charging stability in a severe environment.
Thus, the pulverization process poses a limit in production of finer toner particles as required in higher resolution and higher image quality, and the finer toner particle production is liable to result in remarkable deterioration in uniform chargeability and flowability of the toner.
For overcoming the above-mentioned problems of the pulverization process, the toner production by a suspension polymerization process has been proposed.
A toner produced through suspension polymerization (hereinafter sometimes called a xe2x80x9cpolymerization tonerxe2x80x9d) can be easily provided with a small particle size and is excellent in flowability due to its spherical toner particle shape, thus being advantageous for complying with the requirement for higher image quality.
However, such a polymerization toner is liable to have remarkably lower flowability and chargeability when it contains a magnetic material. This is because generally hydrophillic magnetic particles are liable to be exposed to the toner particle surface. For solving the problem, it is important to modify the surface property of the magnetic material.
Regarding the surface modification of a magnetic material for improved dispersion thereof in a polymerization toner, many proposals have been made. For example, JP-A 59-200254, JP-A 59-200256, JP-A 59-200257 and JP-A 59-224102 have disclosed to treat magnetic materials with various silane coupling agents. JP-A 63-250660 has disclosed to treat silicon-containing magnetic particles with a silane coupling agent. JP-A 7-72654 has disclosed to treat magnetic iron oxide with alkyltrialkoxysilane.
By such treatment, the dispersibility of a magnetic material within a toner is improved to some extent, but uniform surface hydrophobization of a magnetic material is rather difficult. As a result, the occurrence of coalescence of magnetic particles and non-hydrophobized magnetic particles is inevitable, so that the surface modification (hydrophobization) is liable to be insufficient for achieving a good level of dispersibility in the toner.
A special toner containing magnetic particles only at a specific inner portion of particles thereof has been disclosed by JP-A 7-209904, in which, however, no reference is made to the sphericity of the toner particles.
To summarize the toner organization disclosed in JP-A 7-209904, each toner particle has a structure including a surface layer of at least a certain thickness in which no magnetic particles are present. This means that the toner particle includes a substantial surface layer portion containing no magnetic particles. In another expression, this however means that such a toner particle, when in a small average particle size of 10 xcexcm, for example, includes only a small core volume in which magnetic particles are present, so that it is difficult to incorporate a sufficient amount of magnetic particles. Further, in case where such toner particles have a particle size distribution, a large toner particle and a small toner particle have different ratios of magnetic particle-free surface layers and thus different propositions of magnetic particles, so that the developing performance and transferability of the toner particles are different depending on the toner particle sizes, thus being liable to cause a selective development phenomenon depending on particle sizes (i.e., preferential consumption of a certain toner particle size fraction). As a result, if the toner having a certain particle size distribution is used for a long period of continual image formation, toner particles containing a larger proportion of magnetic particles and exhibiting a lower developing ability, i.e., larger toner particles, are liable to remain without being consumed for the development, thus causing lowering in image density and image quality and inferior fixability.
As for printer apparatus, laser beam printers and LED printers are becoming predominant on the market in recent years, and correspondingly, higher resolutions are being desired, e.g., from a conventional level of 240 and 300 dpi to 400, 600 and 800 dpi. For these reasons, the developing scheme is also required to be adapted for higher resolution. Further, also copying machines are required to comply with high functionality copying, and digital-mode copying apparatus are becoming predominant. Along with this trend, the latent image formation by using laser beam is predominant together with a requirement for higher resolution. Accordingly, similarly as in printers, higher resolution and higher definition developing scheme is being required. For complying with such demands, smaller particle size toners having a specific particle size distribution have been proposed in, e.g., JP-A 1-112253, JP-A 1-191156, JP-A 2-214156, JP-A 2-284158, JP-A 3-181952, and JP-A 4-162048.
On the other hand, in recent years when environmental protection is thought much of, a conventional primary charging and transfer process utilizing corona discharge is being gradually shifted to a primary charging and transfer process using a charging member abutted against an electrostatic image-bearing member.
More specifically, in the conventional primary changing and transfer process utilizing corona discharge, a substantial amount of ozone is generated at the time of corona discharge, particularly for generating negative corona, so that an image forming apparatus has to be equipped with a filter for ozone capture, which has required a larger apparatus size and an increased running cost. Such a corona charging scheme has also caused image defects, such as the so-called image flow caused by a lowering in surface resistivity of the photosensitive member due to attachment f ozone adducts, such as nitrogen oxide, and memory of the photosensitive member caused by ions remaining within the charger during the intermission of the image forming apparatus.
For solving the above-mentioned problems encountered in the corona charging system, a contact charging system or a contact transfer system has been developed, wherein a charging member or a transfer member in the form of, e.g., a roller or a blade, is caused to contact a photosensitive member surface to form a narrow space in proximity to the contact portion and cause a discharge presumably according to the Paschen""s law, thereby suppressing the occurrence of ozone to the minimum, e.g., as disclosed in JP-A 57-178257, JP-A 56-104351, JP-A 58-40566, JP-A 58-139156, and JP-A 58-150975. Particularly, a charging scheme and a transfer scheme using an electro-conductive elastic roller as disclosed in JP-A 63-149669 and JP-A 2-123385 have been preferably used in view of the stability.
However, it has been also found that the contact charging system or the contact transfer system is accompanied with a problem to be considered not encountered in the corona discharge system.
More specifically, first in the contact transfer system wherein a transfer member is pressed against a photosensitive member via a transfer paper (i.e., transfer receiving material), at the time of transfer of a toner image on the photosensitive member to the transfer paper, the toner image is compressed thereby to cause a partial transfer failure so-called xe2x80x9chollow imagexe2x80x9d or xe2x80x9ctransfer dropoutxe2x80x9d. Further, as the toner particle size is reduced for complying with a recent demand for a higher resolution and higher definition developing scheme, the forces of attaching toner particles onto the photosensitive member (such as image force and Van der Waals force) become predominant compared with Coulomb force acting on the toner particles for transfer, whereby the transfer residual toner is liable to be increased or the transfer failure is liable to be more serious.
On the other hand, in the contact charging system wherein a charging member is pressed against a photosensitive member surface at a certain pressure, the transfer residual toner is pressed against the photosensitive member surface, so that the photosensitive member surface is liable to be abraded and the toner melt-sticking is liable to be caused at the part of abrasion as a nucleus. This tendency becomes particularly noticeable if the amount of the transfer residual toner is increased.
The occurrence of the abrasion of and toner melt-sticking onto the photosensitive member causes serious defects in electrostatic image formation on the photosensitive member. More specifically, the abrasion of photosensitive member causes a failure of primary charging, so that the part of abrasion results in a black trace in a halftone image. The toner melt-sticking causes a failure of latent image formation by exposure, the part of melt-stuck toner results in a white trace in a halftone image. Further, these defects also deteriorate the toner transferability. Accordingly, in combination with the above-mentioned transfer failure caused by the contact transfer system, remarkable image defects are liable to occur, and the image quality deterioration can be accelerated synergistically in some cases.
The problems of the photosensitive member abrasion and transfer failure are liable to occur especially in the case of using a toner comprising indefinitely-shaped or non-spherical toner particles. This is presumably because of a lower transferability of the non-spherical toner particles and the presence of toner particle edges liable to scratch the photosensitive member surface. Further, the abrasion problem becomes severer in the case of using magnetic toner particles containing a magnetic material exposed to the surface thereof. This may be easily understood in view of a state that the exposed magnetic particles are directly pressed against the photosensitive member.
Further, when the amount of transfer residual toner is increased, it becomes difficult to retain sufficient contact between the contact charging member and the photosensitive member, so that the charging performance is lowered, thus being liable to cause a transfer of toner to non-image portion, i.e., fog in the case of reversal development. This difficulty is liable to be encountered in a low humidity environment wherein the resistivity of the charging member is increased.
As described above, in the image forming system including the contact charging system and the contact transfer system which are very preferable from an ecological viewpoint, it is desirable to develop and use a magnetic toner exhibiting high transferability and less liable to cause photosensitive member abrasion and toner melt-sticking.
On the other hand, in the case where some transfer residual toner remains after a transfer step of transferring a toner image formed on a photosensitive member in the developing step to a transfer-receiving material, the transfer residual toner has to be cleaned and recovered in a waste toner vessel in a cleaning step. In the cleaning step, a cleaning blade, a cleaning fur brush or a cleaning roller has been conventionally used. Any cleaning means has relied on mechanically scraping off or damming the transfer residual toner for recovery into the waste toner vessel. However, the use of such a mechanical cleaning means wears and shortens the life of the photosensitive member. From the apparatus viewpoint, the presence of cleaning device has posed an obstacle to provision of a compact apparatus. Further, from the viewpoints of ecology and effective toner utilization, a system free from generation of waste toner, i.e., a cleanerless system, is desirable.
Such cleanerless image forming systems have been discussed in JP-A 59-133573, JP-A 62-203182, JP-A 63-133179, JP-A 64-20587, JP-A 2-302772, JP-A 5-2289, JP-A 5-53482 and JP-A 5-61383. Moreover, a serious attention has not been paid to a desirable toner organization to be used in such cleanerless image forming systems.
JP-A 61-279864 has proposed a toner having specific shape factors SF-1 and SF-2, no reference is made to a transfer step using the toner. Further, as a trace experiment of ours, the toner exhibited a toner efficiency which is low and therefore has left a room for improvement.
JP-A 63-235953 has disclosed a magnetic toner sphered by mechanical impact, but the transfer efficiency thereof is still low and has left a room for further improvement.
Incidentally, a cleanerless image forming system including a simultaneous developing and cleaning scheme, a photosensitive member surface is rubbed with a toner and a toner-carrying member for recovering a toner on a non-image portion and supplying a toner to an image portion on the photosensitive member by the toner-carrying member. At the time of rubbing, if reversibly charged toners inclusive of transfer residual toner and fog toner can be oppositely charged to a normal polarity, such toners can be potentially easily recovered.
As a result of our study, in case where a conventional toner containing a magnetic material is used in such an image forming system including a simultaneous developing and cleaning scheme, a partial electrical continuity is caused at the time of developing between the photosensitive member and the tone-carrying member via the toner due to the magnetic material exposed to the toner particle surface, so that the electrostatic latent image on the photosensitive member is disturbed thereby and it is difficult to obtain a high definition image. Further, such a magnetic toner containing a magnetic material exposed to the toner particle surface causes an insufficient charge of the transfer residual toner, so that the smooth recovery thereof from the photosensitive member during the developing step is obstructed. Further, at the time of rubbing of the photosensitive member with the toner and the toner-carrying member, the photosensitive member is liable to be severely worn due to the magnetic material exposed to toner particle surface, thus shortening the life of the photosensitive member. As a result, there results in a so-called ghost image, i.e., a soiling toner images attached onto a non-image region.
Accordingly, in an image forming system including a simultaneous developing and cleaning scheme, a magnetic material-containing toner is desired to be free from exposure of the magnetic material to the toner particle surface.
Further, in an image forming system retaining a cleaning member while including a simultaneous developing and cleaning scheme, if the abutting pressing of the cleaning member against the photosensitive member is lowered in order to retain a longer life of the photosensitive member, an increased amount of the transfer residual toner can slip by the cleaning member to reach the developing step. In such a system, it is also very important to minimize the amount of the transfer residual toner stipping by the cleaning member even under a reduced abutting pressure of the cleaning member.
The above-mentioned problems encountered in the case of using a conventional magnetic material-containing magnetic toner have been principally caused by the exposure of the magnetic material to the toner particle surface. As another factor, in the case of a magnetic toner containing a magnetic material exposed to the toner particle surface, the magnetic toner is liable to have an unstable chargeability in a high humidity environment due to a lower resistivity of the magnetic material than the toner binder resin, thus causing difficulties, such as increased fog, lower transferability and a lower recovery rate of the transfer residual toner leading to the occurrence of ghost images, in addition to the performance deterioration of the photosensitive member due to abrasion of the photosensitive member by rubbing with the exposed magnetic material.
In view of the above factors, a magnetic toner exhibiting good initial performances and stability of performances in an image forming system including a simultaneous developing and cleaning scheme has not been obtained as yet.
A generic object of the present invention is to provide a toner and an image forming method having solved the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a magnetic toner which exhibits stable chargeability, less susceptible of environmental changes, and can provide images having high image density and with suppressed fog at a good image reproducibility even after a long period of continual use.
Another object of the present invention is to provide an image forming method which has solved the above-mentioned problems in the image forming process based on the contact development-scheme capable of omitting a cleaner system and can provide images free from fog and ghost with excellent resolution, transferability and excellent durability without being affected by environmental conditions.
Another object of the present invention is to provide an image forming method including a contact changing step of less ozone-generation type and a non-contact developing method using a magnetic toner (mono-component developer) providing images with less fog, wherein a magnetic toner exhibiting good transferability to cause less transfer dropout and less transfer residual toner and less abrading the photosensitive member, thus being less liable to result in image defects even after a long period of continual use.
Another object of the present invention is to provide an image forming method capable of stable electrostatic latent image formation even in a low humidity environment and resulting in less image defects such as fog due to a lowering in chargeability in continuous image formation.
According to the present invention, there is provided a toner, comprising: toner particles each comprising at least a binder resin and iron oxide particles, wherein
(i) the toner particles exhibit a carbon content (A) and an iron content (B) giving a ratio B/A less than 0.001 at surfaces of the toner particles as measured by X-ray photoelectron spectroscopy,
(ii) the toner particles exhibit an average circularity of at least 0.970, and
(iii) the toner particles contain at least 50% by number of toner particles satisfying D/Cxe2x89xa60.02, wherein C denotes a projection area-equivalent circular diameter of each toner particle and D denotes a minimum distance of iron oxide particles from a surface of the toner particle, based on a sectional view of the toner particle as observed through a transmission electron microscope (TEM).
According to another aspect of the present invention, there is provided an image forming method, comprising:
a charging step of charging an electrostatic image-bearing member with a charging member receiving a voltage from an external voltage supply,
an exposure step of exposing the electrostatic image-bearing member to form an electrostatic latent image thereon,
a developing step of developing the electrostatic latent image with the above-mentioned toner carried on a toner-carrying member to form a toner image on the electrostatic image-bearing member, and
a transfer step of transferring the toner image onto a transfer-receiving material.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.